zend 2：使用Zend \ Form查看模板中的变量容器

I'm reading a book on ZF2 "Using Zend Framework 2" and saw something that confused me.

It was an example of using Zend\Form in a standard MVC style app where a variable is injected into a view like normal:

return new ViewModel(array(
    'form' => $form
));

But in the template it does this:

<?php
   $form = $this->form;
   $form->prepare();
?>

Why is $this->form being assigned to a local variable? I can just do $form->prepare() and it looks like it works. If I just call $form->prepare() does it not permanently modify the $form variable container? Do you have to copy it to a local variable first before calling prepare()?

thanks for reading my book. This is just for convenience. The $form->prepare() looks shorter than $this->form->prepare(). These two ways are absolutely equivelent.